While looking at the information regarding how to mod attachments on p. 187 it doesn't seem to state whether implementing a modification option requires the use of an additional HP, only that it increases the difficulty of the Mechanics check by one and costs an add'l 100 credits beyond the base cost.

Is that correct, that improving a mod does not require the use of an add'l HP as it is just augmenting the attachment itself?

Mods don't take up HP, only the Attachment you're modifying does that.

The other limitations have, in my group at least, proven sufficient to stop things getting silly.